When the pressure of oil or gas in a reservoir declines as oil or gas is taken from that reservoir, production from a well in that reservoir declines and the economic viability of the well declines until it is no longer profitable to operate (even though it continues to produce gas or oil). Production can be increased from such wells through oil well stimulation. In addition, where forming a bore hole into a reservoir is very expensive, such as in offshore drilling, it is desirable to stimulate production from a single well.
Oil well stimulation typically involves injecting a fracturing fluid into the well bore at extremely high pressures to create fractures in the rock formation surrounding the bore. The fractures radiate outwardly from the well bore, typically from 100 to 1000 meters, and extend the surface area from which oil or gas drains into the well. The fracturing fluid typically carries a propping agent, or “proppant”, such as sand, so that the fractures are propped open when the pressure on the fracturing fluid is released, and the fracture closes around the propping agent. This leaves a zone of high permeability (the propping agent trapped and compacted in the fracture in the subterranean formation.
The fracturing fluid typically contains a water soluble polymer, such a guar gum or a derivative thereof, which provides appropriate flow characteristics to the fluid and suspends the proppant particles therein. When pressure on the fracturing fluid is released and the fracture closes around the propping agent, water is forced therefrom and the water-soluble polymer forms a compacted cake. This compacted cake can prevent oil or gas flow if not removed. To solve this problem, “breakers” are included in the fracturing fluid.
Currently, breakers are either enzymatic breakers or oxidative breakers. The enzyme breakers are preferable, because (a) they are true “catalysts”, (b) they are relatively high in molecular weight and do not leak off into the surrounding formation, and (c) they are less susceptible to dramatic changes in activity by trace contaminants. Oxidative breakers, on the other hand, are low in molecular weight and leak off into the formation, and they are active only over a very narrow temperature range. Enzyme breakers, however, are inactive at higher temperatures, limiting their use to shallow wells. It would accordingly be highly desirable to have enzyme breakers that operate at higher temperatures to enable fracturing of deep wells. See generally J. Gulbis, Fracturing Fluid Chemistry, in RESERVOIR STIMULATION, Chap. 4 (J. J. Economides and K. G. Nolte, Eds., 2d Ed. 1989).
U.S. Pat. No. 4,996,153 to Cadmus and Slodki discloses a heat-stable enzyme breaker which may be used as a viscosity breaker in oil recovery, but this breaker is a xanthanase for degrading xanthan-based rather than guar-based fracturing fluids, and is only said to be active at 55° C. (156.6° F.).
U.S. Pat. No. 5,201,370 to Tjon-Joe-Pin discloses enzyme breakers for galactomannan-based fracturing fluids, which enzyme breakers are galactomannases that hydrolyze the 1,6-α-D-galactomannosidic and the 1,4-β-D-mannosidic linkages in the guar polymer, but these are said to only be active at low to moderate temperatures of about 50° F. to 180° F.
U.S. Pat. No. 4,250,044 to Hinkel concerns a tertiary amine/persulfate breaker system, and not an enzyme system.
In view of the foregoing, there is a continued need for thermostable enzyme breakers useful for fracturing subterranean formations in the course of oil and gas well stimulation.